1. Field of the Invention
The present invention relates to a lightning arrestor used to protect an electronic device from an abnormal voltage generated in a power supply system.
2. Description of the Related Art
Typically, a polymer type lightning arrestor used in an electric power station or a transformer substation includes internal components including a stack of zinc oxide components, terminal electrodes disposed on both ends of the stack of zinc oxide components, and a plurality of insulation supports made of glass fiber-reinforced plastic (FRP) or the like for connecting the terminal electrodes to each other. A polymer type lightning arrestor is formed by directly molding an insulation material such as a polymer into an insulation casing around these internal components.
In a short circuit test of the above-detailed lightning arrestor according to the IEC standard 60099-4, the zinc oxide component is failed by injecting an amount of energy that is larger than the zinc oxide component can tolerate. In this state, when a short-circuit current is flown through the lightning arrestor, a high-pressure high-temperature arc gas is generated in the lightning arrestor. As the inner pressure of the lightning arrestor increases due to the arc gas, the internal components may be explode and scatter within the lightning arrestor. However, the arc gas can be released outside of the lightning arrestor instantaneously if the insulation casing is opened before the inner pressure increases.
Japanese Patent Application Laid-open No. H10-162927 discloses a current-limiting unit as an exemplary technique for instantaneously releasing the arc gas generated in the lightning arrestor that has been short-circuited due to an excessive lightning surge. Japanese Patent Application Laid-open No. H10-162927 (see FIGS. 3 and 5), a thin-thickness portion having a smooth curvilinear surface such as a circular surface or an elliptical surface having a large curvature radius is provided on a part of the outer circumferential surface between the corrugations of a cylindrical insulation casing of a current-limiting unit. The thin-thickness portion is opened instantaneously and serves as a pressure-releasing portion to discharge the arc gas when the lightning arrestor receives an excessive lightning surge. In addition, the thin-thickness portion is provided to face a different direction from any neighboring cable support insulators in order to avoid the cable support insulators being damaged by the released arc gas.
Japanese Patent Application Laid-open No. 2003-92205 discloses a lightning arrestor having improved mechanical strength by supporting internal components with an insulation support such as fiber reinforced plastic (FRP). In this lightning arrestor, because the insulation material and the insulation support are integrally molded into an insulation casing, the freedom of the shape of the insulation casing is limited. In other words, it is difficult to provide a thin-thickness portion such as a circular surface or an elliptical surface having a large curvature radius as described in Japanese Patent Application Laid-open No. H10-162927.
The current-limiting unit disclosed in Japanese Patent Application Laid-open No. H10-162927 (see FIGS. 1 and 4) is used in a place where no load is applied in the lightning arrestor, as shown in FIG. 7 of the same document. In other words, it fails to provide any mechanical strength to support the zinc oxide component with an insulation casing made of polymer or the like by bonding the terminal electrode and the zinc oxide component. Moreover, this current-limiting unit has a structure similar to a distributor type lightning arrestor having a low current level. Therefore, the existing current-limiting unit also fails to satisfy the mechanical strength required in the support structure described above when it is used in, for example, a transformer substation.
In addition, the support structure disclosed in Japanese Patent Application Laid-open No. H10-162927 (see FIGS. 1 and 4) does not have a structure for preventing scattering of shattered portions that may be generated when the zinc oxide component is failed by the short-circuited lightning arrestor. Therefore, the shattered portions may be scattered away from the opening of the insulation casing as soon as the arc gas is released.
On the other hand, in the structure shown in FIG. 3(c) of Japanese Patent Application Laid-open No. 2003-92205, a part of or all of the spaces between the insulation supports of the insulation casing are opened in order to release pressure when an arc gas is generated by a short-circuit current flowing through the lightning arrestor. In this case, because there is no thin-thickness portion, it is difficult to control the pressure-releasing direction, and thus, the arc gas is released in an arbitrary direction. As a result, neighboring devices may be damaged by the released arc gas depending on the direction of release of the arc gas.